In the cryogenic separation of air, feed air is compressed, cooled and then a portion usually isenthalpically or isentropically expanded to effect further cooling and thereby generating a liquid. The liquid typically is charged to a double distillation column having a high pressure and a low pressure section wherein the resulting liquidfied air is separated. High purity oxygen is generated in the low pressure section and withdrawn as a liquid. Often argon is recovered as a component and typically that will involve removing an oxygen enriched sidestream from the low pressure column to an argon column for further separation. Other separation cycles including single columns also are used for the separation of air into its components.
Recently there has been substantial activity in improving the distillation process associated with the separation of oxygen and nitrogen through the use of packings as opposed to trays in the distillation column. Structured packings have been suggested for the purpose of facilitating separation with reduced pressure drop in the column.
Representative patents which describe air separation processes using structured packing for enhancing distillation through enhanced mass transfer with reduced pressure drop in the column are as follows:
U.S. Pat. No. 4,836,836 discloses an improved cryogenic distillation system for separating air into its components. The invention resided in replacing trays within a distillation column with structured packing in those regions of the distillation column where the concentration of argon was in the range from about 0.6 to about 75% by volume. The general suggestion was to substitute packing for the trays in both the high pressure and low pressure section of the column and particularly substituting the packing for trays in a low pressure argon column. There were two primary reasons for the substitution of packings for these trays, one, there was enhanced separation through greater mass transfer, e.g., more theoretical stages for a given space; and two, there was a greater number of stages with reduced or equivalent pressure drop. In the low pressure argon column, minimizing pressure drop across the column was extremely important, because it controls the pressure and thus the bubble point of the reboiling stream.
U.S. Pat. No. 4,838,913 discloses the cryogenic separation of air and recovery of argon in a side column. The patentees point out that vapor-liquid contact is achieved in a low pressure section of a double column utilizing packing in a section of the low pressure column below the intermediate point, i.e., the point relating to the point of removal of gases for argon recovery in the argon column.
The utilization of thin metal sheet for the fabrication of structured and random packing for use in distillation columns designed for the separation of air into its components has presented problems in terms of the selection of materials of construction for packing materials in an oxygen enriched environment. It was known from prior art processes that as oxygen concentration increased, certain materials would not be suited for use as the material of construction. Conventional wisdom suggested copper, brass and stainless steel as being acceptable materials while aluminum was not a recommended material, particularly where aluminum components have a thickness of less than about 0.8 millimeters. Representative literature suggesting suitable materials of construction for trays, packing, etc. for a distillation column used by air separation include:
U.S. Pat. No. 4,813,988 discloses a variety of packing materials for air separation processes and materials of construction for such packings include carbon steel, stainless steel, aluminum, copper alloys and plastics. The general suggestion was that a packing element fabricated from a copper containing material having a concentration of copper of at least about 30% by weight is preferred because of its resistance to flammability. The patentees point out that packing elements present additional problems because of the relatively thin nature of the material and the thin film of liquid that may be retained on the surface of the material.
ASTM Designation G94-88 standard guide for evaluating metals for oxygen service pages 12-20 provides and excellent knowledge base for selecting metals which are suitable and unsuitable for direct oxygen service. Copper and brass are suggested as being preferred.